This PDF file contains the front matter associated with SPIE Proceedings Volume 8246, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

We discuss the use of photon-counting array receivers for communications links employing on-off-keyed and frequencyshift- keyed modulation formats. The effects of detector non-idealities, such as reset time and timing resolution on achievable receiver performance are presented.

Coherent states achieve the Holevo capacity of a pure-loss channel when paired with an optimal measurement, but a physical realization of this measurement scheme is as of yet unknown, and it is also likely to be of high complexity. In this paper, we focus on the photon-counting measurement and study the photon and dimensional efficiencies attainable with modulations over classical- and nonclassical-state alphabets. We analyze two binary-modulation architectures that improve upon the dimensional versus photon efficiency tradeoff achievable with the state-of-the-art coherent-state on-off keying modulation. We show that at high photon efficiency these architectures achieve an efficiency tradeoff that differs from the best possible tradeoff--determined by the Holevo capacity--by only a constant factor. The first architecture we analyze is a coherent-state transmitter that relies on feedback from the receiver to control the transmitted energy. The second architecture uses a single-photon number-state source.

Current development status of the small optical transponder (SOTA) to be installed into a small satellite is described, where the breadboard model, the engineering model and the protoflight model are respectively introduced. The tracking performance is estimated to show that the angular error is low enough in comparison to the divergence angle of the transmitted beam.

An ideal intensity-modulated photon-counting channel can achieve unbounded photon information efficiencies (PIEs). However, a number of limitations of a physical system limit the practically achievable PIE. In this paper, we discuss several of these limitations and illustrate their impact on the channel. We show that, for the Poisson channel, noise does not strictly bound PIE, although there is an effective limit, as the dimensional information efficiency goes as e^-ePIE beyond a threshold PIE. Since the Holevo limit is bounded in the presence of noise, this illustrates that the Poisson approximation is invalid at large PIE for any number of noise modes. We show that a finite transmitter extinction ratio bounds the achievable PIE to a maximum that is logarithmic in the extinction ratio. We show how detector jitter limits the ability to mitigate noise in the PPM signaling framework. We illustrate a method to model detector blocking when the number of detectors is large, and illustrate mitigation of blocking with spatial spreading and filtering. Finally, we illustrate the design of a high photon efficiency system using state-of-the-art photo-detectors and taking all these effects into account.

Single polarization high-speed optical transmission is important for bidirectional free-space optical communication system in order to have enough isolation up-link and down-link by signal discrimination using orthogonal polarization states. At recent advance in digital coherent technology, polarization re-combining in combination with polarization diversity receiver is widely used to suppress performance degradation when polarization states of signal and local oscillator are misaligned by system vibration or shocks. However, in order to implement the re-combining function in digital signal processing, appropriate algorithm is required for realizing the system stability. In this paper, we demonstrate a new algorithm implementation for single polarization receiver with maximal-ratio-combining (MRC) technique. First we exhibited the problem for state-of-the-art polarization re-combining in instability due to singularity condition at 45-degree-azimuth elliptic polarization. In order to overcome this problem, we proposed a newly MRC algorithm added splitting ratio dependent phase correction coefficients and achieved stable re-combining at 45-degreeazimuth elliptic polarization signal. And we successfully demonstrated the stable receiving for 50-Gb/s single polarization QPSK signal with all polarization states by our digital coherent receiver platforms added the newly MRC algorithm, compared with previous-proposed MRC algorithm.

The time slots of M-ary pulse-position-modulation (PPM) can be replaced by M optical CDMA codes: Code/Pulse-Position-Swapping (C/PPS) or Code-Shift-Keying (CSK). Features of C/PPS are that it: (1) retains the intensity-modulation/ direct-detection (IM/DD) and multiple-bits-per-symbol features of PPM; (2) supports multiple access communications; (3) supports bandwidth-on-demand; (4) supports tailoring quality of service; (5) translates the received symbol to its corresponding bit sequence without using look-up-tables (LUTs)); (6) and has a common electronic bandwidth for all data rates. In this paper we describe the design of C/PPS encoders and decoders based on the specific example of 4x4 fast frequency hopping (FFH) sonar codes.

The Lunar Laser Communications Demonstration (LLCD), a project being undertaken by MIT Lincoln Laboratory, NASA's Goddard Space Flight Center, and the Jet Propulsion Laboratory, will be NASA's first attempt to demonstrate optical communications between a lunar orbiting spacecraft and Earth-based ground receivers. The LLCD space terminal will be flown on the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft, presently scheduled to launch in 2013. LLCD will demonstrate downlink optical communications at rates up to 620 Mbps, uplink optical communications at rates up to 20 Mbps, and two-way time-of-flight measurements with the potential to perform ranging with sub-centimeter accuracy.

For the first time, laser communication terminals will be operational in a commercial service, EDRS, the European Data Relay System. Future earth observation satellites call for satellite-to-ground links with high availability to make their data immediately available to the user. High availability is only achieved by GEO relay links, i.e. communication links from LEO satellites to a geostationary relay satellite which transmits the data to the ground. While RF communication limits the GEO relay's data rate to roughly 1 Gbps optical communication will extend its capacity into the 10 Gbps range. Today, laser communication terminals operated in LEO-LEO, LEO-to ground and ground-to-LEO links, all at a data rate of 5.625 Gbps, are applied in campaigns with scientific objectives and for technology demonstrating purposes. Adapted to the larger link distance of the LEO-to-GEO link the laser communication terminals will be applied in the GEO relay EDRS operated for the time being at 1.8 Gbps. Terminals will be delivered for the accommodation on Alphabus, EDRS-A and EDRS-C, European GEO-satellites, and on Sentinel-1a and Sentinel-2a, European LEO satellites.

JPL is developing and testing a 10 Gb/s laser communications terminal for use with earth-orbiting spacecraft. The system consists of an optical transceiver head on a two-axis gimbal with a separate electronics assembly that contains the avionics, modem and laser. The link is achieved by multiplexing on the flight terminal four 0.5-W lasers in the C-band that are modulated at up to 2.5 Gb/s; transmitted through a 5-cm aperture; and received by a 1m ground aperture. A description of the system together with experimental tests of a prototype is presented.

Advantages of optical links like small, lightweight and power efficient terminals are practical for high data rate services of disaster preparedness and environmental research. In this paper, we demonstrate experimental results of 40-Gbit/s optical free space transmission using single-polarization quadrature phase shift keying (SP-QPSK) modulation format and digital coherent detection. The digital coherent detection enabled a high sensitivity and a tolerance to transmission impairments, which have attractive features for free space transmission system. We developed a 50-Gbit/s SP-QPSK transmitter and offline-receiver with the optical antenna system. SP-QPSK optical modulation signal with a line rate of 50-Gbit/s including 20% FEC is employed for high receiver sensitivity. A cascade by EDFAs consisting of a low noise pre-EDFA and an optical level controlled EDFA is developed to compensate for level fluctuation without degrading receiver sensitivity. Maximal ratio combining algorithms and carrier phase estimation algorithms are used at the offline-receiver for QPSK signal detection. We succeeded 4-meter indoor free space transmission having same performance as that with fiber connection using the developed system. The optical received power was -42 dBm at bit error rate of 10^-3. While for outdoor 50-meter transmission, we confirmed the received bit error rate larger than FEC limit.

The study of the free-space propagation of quantum correlations is necessary for any future application of quantum communication aiming to connect two remote locations. Here we study the propagation of a coherent laser beam over over 144 Km (between Tenerife and La Palma Islands of the Canary archipelagos). By attenuating the beam we also studied the propagation at the single photon level. We investigated the statistic of arrival of the incoming photons and the scintillation of the beam.

We demonstrate environmental test results of a compact high-power master oscillator (MO) power amplifier (PA) laser module emitting near 1064 nm. The module is micro-integrated on a footprint of 50 × 10mm², features an output power of 1 W, an intrinsic linewidth of 3.6 kHz, and a FWHM linewidth of 100 kHz. Results of lifetime tests and vibrational tests will be presented. Furthermore, we introduce tuning characteristics of a distributed Bragg reflector laser that can be used as a master oscillator. 2 nm of fast tuneability is realized by placing on-chip heaters close to the Bragg grating.

There is demand for improved deep-space satellite communications links with increased data rates to accommodate new sensor technologies and increased sensor payloads on spacecraft. It is imperative that new solutions be compact in size, light in weight, be high speed, and highly power efficient. Optical links offer potential improvements in power, size and weight due to a substantially narrower beam and smaller components. Solutions using fiber-laser transmitter master-oscillator power-amplifiers (MOPA) have been investigated previously, but methods for improving the system power efficiency are needed. In this paper we will present recent results for a 1.5um fiber MOPAs for deep-space communications. A high-power, broad linewidth 1550 nm seed laser is combined with 3 stages of amplification. Each of these stages is partially designed based on the availability of the various components An amplifier operating at 1532 nm in principle has the advantage of commercial off-the-shelf components with demonstrated reliability though the availability of high power broad area pump diodes at 1532nm with good efficiency is limited. The final power amplifier stage was constructed using both step index fiber (SIF) and photonic crystal fiber (PCF).

KW level fiber based MOPA laser system at 1018-1024nm for uplink deep space communication is developed and characterized. System achieves 5kW peak power (300W average power) with >75% optical conversion efficiency at 1024nm. Experiments with 9-ary PPM pulse format at 500 kHz are presented. Pattern dependent pulse energy variation <10% is achieved using pulse pre-shaping algorithm. Lower PRF performance of the system where ASE accumulation along MOPA chain results in reduction in efficiency is characterized. Excellent agreement is established between dynamic fiber MOPA simulation tool and experimental results in predicting pulse shapes and ASE accumulation. Using the simulation tool robust MOPA performance is predicted at 500W, 500 kHz (16-ary PPM ) for the current system with minor component improvements. Simulation tool is also shown to be well suited for quantifying statistical performance of open loop pulse shape correction algorithms.

Pulsed fiber based laser transmitters suitable for optical communications in both near Earth and deep space scenarios are being developed in partnership between JPL and industry. As a precursor to a full space qualification process, commercial components were integrated into a polarization maintaining 1550 nm master oscillator laser as part of a master oscillator power amplifier (MOPA) laser transmitter designed to support pulse position modulation and operation in the vacuum environment. This optically pre-amplified seed laser generated sub-ns pulses at up to 100 MHz with high extinction ratio over variable duty cycle. Test results in both ambient and under vacuum conditions will be reported for this system including a preliminary life-test with over 5000 hours under high vacuum at temperatures ranging from 0 to 40 °C.

A ground-based 40W 1.55μm uplink transmitter for lunar laser communications is described. The transmitter, which generates wavelength multiplexed communication and beacon signals, is implemented using four 10W spatial-diversity channels to reduce far-field atmospheric-turbulence-induced fading and facilitate high-power signal generation via parallel-spatial-combining of commercially-available EDFAs. Each transmitter channel can generate a 1 kHz modulated beacon for spatial acquisition, and a multi-rate 4-PPM communication signal at a 311 MHz slot rate with 16:1 and 32:1 duty cycles to support 38.9 Mbit/s and 19.4 Mbit/s channel rates, respectively. Details on the transmitter design, including the mitigation of optical nonlinear effects are discussed.

To explore anisoplanatism over horizontal paths a LCS experiment was developed. The experiment operated in real-world conditions over a 3.2km path, partly over water. To compare the results obtained from the experimental data and established theory, we modeled the experimental path via simulation using a finite number of phase screens. The scale and location of the phase screens in the simulation were varied to account for a different turbulence conditions along the propagation path. Preliminary comparison of our experimental data and simulation show that adjacent PSFs are significantly correlated at angles much larger than the predicted theoretical isoplanatic angle.

New communication systems are emerging with growing demand on the data traffic. Fiber optics allow us to transfer data of rates higher than hundreds gigabits per second. These systems are usually used for backbone networks, although using of these systems is appearing in last mile connections. Wireless communication systems are still more and more popular owing to fast and cheaper deployment contrary to wire or fiber communication systems. On the other hand, wireless communication systems including free-space optics (FSO) are affected by weather conditions. In order to mitigate these effects, several mitigation techniques were investigated. For instance spatial diversity using more than one transmitter or receiver, auto-tracking of an optical beam, wavelength diversity utilizing radio band links as a backup etc. FSO links may be used in FSO networks to connect several building of a corporation, hospital or university campus. In case of proper network topology, route diversity can contribute to availability enhancement. In this paper, mitigation technique based on the route diversity is presented. Results are compared with hybrid FSO link.

The impact of clouds on optical Earth-space links is assessed by means of theoretical analyses. The spacecraft is assumed geostationary so that a stationary model can be considered for the link availability. Binary and continuous attenuations are analysed separately. With a network of ground stations, both spatial diversity and multiplexing are considered. The impact of the ground-station network size and spatial correlation on the availability and throughput is studied. Using a continuous attenuation model, it is shown that an adaptive data rate essentially mitigates thin clouds (on the order of 10-dB attenuation).

Reception of faint optical communications signals from deep space presumes multi-meter diameter optical receivers coupled to high detection efficiency photon counting detectors. Superconducting nanowire detectors presently offer the highest performance for photon starved optical communications at near-infrared optical communications wavelengths. Square-millimeter sized arrays are required due to atmospheric turbulence and the classical solid angle-area invariant of an optical system, but most development of superconducting nanowire detectors has been for small pixels and arrays of less than 0.001 square-millimeter area. One deep space receiver approach is to partition detector area across multiple receive apertures (multiple telescopes) to use these small detectors, but this carries performance and cost penalties compared to use of a single large aperture. At JPL we are pursuing the development of large superconducting nanowire arrays for free space coupling to multi-meter telescopes and have developed a facility for testing large area free-space coupled nanowire arrays and fabricated arrays up to 64 pixels in the NbTiN and W_(1-x)Si_x material systems.

This paper reports a new beam setting for single-mode-fiber coupled free-space laser communication terminals with bidirectional beacon tracking. To maintain stable and robust laser beam transmission over 0.2-2km terrestrial horizontal link, optimum beam setting is found to be not a collimated beam but a symmetric confocal beam between the two terminals. The paper also describes the detail of optics, such as optical antenna and fiber coupling lens used in the compact free-space optical terminals.

Conical refraction (CR) is proposed to increase the channel capacity for free space optical communication applications. We present the first investigations of cascaded CR with a linearly polarized input beam and experimentally prove that two oppositely oriented consecutive identical biaxial crystals perform a forward-backward transformation of the incident light beam. This forward-backward transformation is reported for different input beams with Gaussian, elliptical and angularly modulated transverse intensity profiles and is the basis for our novel proposal on multiplexing and demultiplexing of optical beams. We present experimental proof of usefulness and perspective of the CR multiplexing technique by increasing in one order of magnitude the channel capacity at optical frequencies. The technique is applicable to any wavelength in optical and telecommunication bands. It can be also properly upgraded with the WDM technique.

The long-term throughput of optical links between a circular-LEO satellite and a ground station is assessed for systems with fixed and variable data rates. For a fixed data rate, we evaluate the minimum elevation angle above which the link should be established in order to maximize the throughput. The ratio of this optimized constant-data-rate throughput to the throughput of an adaptive transmission with a data rate proportional to the received power is found to depend mainly on the satellite altitude.

A number of space agencies, including NASA, are considering free-space laser communications as a means for returning higher data-rates from future space missions. In this paper, potential deep-space missions are evaluated to show that with optical communication a 10× increase relative to state-of-the art telecommunication systems could be achieved. The maximum deep-space distance where ground transmitted laser beacons could assist acquisition and tracking; and operating points where optical communication performance degrades faster than the inverse square distance are also discussed.

This paper was presented at the SPIE conference indicated above and has been withdrawn from publication at the request of the authors.

Er/Yb doped fibers and amplifiers have been shown to be very radiation sensitive, limiting their integration in space. We present an approach including successive hardening techniques to enhance their radiation tolerance. The efficiency of our approach is demonstrated by comparing the radiation responses of optical amplifiers made with same lengths of different rare-earth doped fibers and exposed to gamma-rays. Previous studies indicated that such amplifiers suffered significant degradation for doses exceeding 10 krad. Applying our techniques significantly enhances the amplifier radiation resistance, resulting in a very limited degradation up to 50 krad. Our optimization techniques concern the fiber composition, some possible pre-treatments and the interest of simulation tools used to harden by design the amplifiers. We showed that adding cerium inside the fiber phosphosilicate-based core strongly decreases the fiber radiation sensitivity compared to the standard fiber. For both fibers, a pre-treatment with hydrogen permits to enhance again the fiber resistance. Furthermore, simulations tools can also be used to improve the tolerance of the fiber amplifier by helping identifying the best amplifier configuration for operation in the radiative environment.